1. Field of the Invention
The present invention relates to methods and apparatus for electrically heat welding thermoplastic fittings, and more particularly, to improved methods and apparatus for automatically electrically heat welding thermoplastic fittings to other thermoplastic members such as pipe sections.
2. Description of the Prior Art
Electric heat weldable fittings formed of thermoplastic material have been developed and used heretofore. Such fittings generally include an electric resistance heating coil or element positioned adjacent the inside surfaces of the fitting which are to be welded to one or more other thermoplastic members such as pipe joints. The electric resistance heating element is usually a coil of resistance wire disposed in the thermoplastic material of the fitting. Electric contacts are attached to an outside surface of the fitting so that a source of electric power can be applied to the heating coil. Examples of electric heat weldable thermoplastic fittings are described in U.S. Pat. No. 4,147,926 issued Apr. 3, 1979 and U.S. Pat. No. 4,349,219 issued Sept. 14, 1982.
When electric power is applied to the resistance heating element of a fitting, the heating element heats the fitting and adjacent portions of thermoplastic members to temperatures which cause the thermoplastic materials from which the fitting and adjacent members are formed to melt whereby they become fused or welded together. The quality of the weld which results is primarily dependent upon the correct quantity of electric power being supplied to the heating element of the fitting. If too little electric power is supplied, an inadequate low strength weld results. If too much electric power is supplied, the fitting and adjacent members to which the fitting is welded can be overheated and deformed whereby a good weld does not result.
Various types of control and electric power generating apparatus have been utilized for supplying the electric power to electric heat weldable thermoplastic fittings. Initially, such apparatus were manually controlled by an operator and the quantities of electric power supplied to the heating elements of the fittings were determined by visual observation. More recently, control apparatus have been developed whereby the particular heating element and fitting to be welded are identified and electric power is supplied to the heating element in a preset quantity. For example, U.S. Pat. No. 4,602,148 issued July 22, 1986 and U.S. Pat. No. 4,631,107 issued Dec. 23, 1986, describe methods and apparatus which automatically identify the particular heating element and fitting to be welded, a controlled quantity of electric power is supplied to the heating element of the fitting and the welding process is monitored so that if an abnormality therein is detected, the welding process is terminated.
Improvements have been made to the methods and apparatus for electrically heat welding thermoplastic fittings of the type described above. For example, U.S. Pat. No. 4,642,155 issued Feb. 10, 1987 and U.S. Pat. No. 4,684,789 issued Aug. 4, 1987, disclose methods and apparatus which initially measure the resistance and/or impedance of the heating element using electric power at a minimum voltage level which does not significantly heat the element to more accurately determine the size of the fitting to be welded. In addition, the electric power supplied to the heating element of the fitting during the welding process is alternating current electric power which by adjusting the frequency thereof can be made to cause the vibration of the fitting which improves the quality of the weld obtained.
A problem which has not yet been solved in the electric heat welding of thermoplastic fittings involves the situation where the heating element of the fitting is partially shorted out as a result of improper installation or other reason. When electric power of substantially constant voltage is supplied to such heating elements and when a short circuit develops, the resistance of the heating element decreases which causes the current level to increase, often to a level whereby the ignition point of the thermoplastic material is reached and a fire or other dangerous condition occurs. While an automatic electric power apparatus of the type described above which determines abnormalities in the welding process terminates the welding process when a high current level is experienced, the end result is that the fitting and adjacent portions of pipe joints or the like must be replaced and the welding thereof repeated.
By the present invention improved methods and apparatus for electrically heat welding thermoplastic fittings to adjacent thermoplastic members are provided whereby when a short circuit in the heating element occurs, the current level is prevented from rising appreciably and controlled at a predetermined current level to prevent overheating and, in most cases, to complete the welding process whereby an acceptable weld is made.